This invention relates to an optical data recording media which is provided with a recording layer of which optical characteristics are changed by the difference of heat history of heating by the irradiation of the laser light and cooling, and records, erases and reads data by the irradiation of the laser light to this recording layer.
An optical disk which records and reads data by utilizing the laser beam is worthwhile to draw attention as a file memory which has a large storage capacity and is portable. As this type of optical disks, read-only optical disk, write-once optical disk and rewritable disk (overwritable) have already been commercialized. Among these optical disks, phase change optical disk, exchange coupled overwritable disk and magnetic field modulation optical disk are known as the overwritable system.
In the phase change optical disk, data can be recorded or erased by utilizing the property of recording layer that locally changes the optical characteristics by the difference of heat history, when that part of the recording layer which is irradiated by the laser beam receives the heat history of heating by the irradiation of the laser beam and cooling of the recording layer. That is, in this phase change optical disk, the energy of irradiation light is modulated between the powers corresponding to two optically distinguishable states. Thereby, data can be recorded or new data can be overwritten over old ones. In this case, the high power energy of irradiating light is referred as the recording power and the low power energy as the erase power.
Also, as the method for aligning the new data to be recorded and the recording mark on the recording media, i.e., the modulation method, there is a pulse position modulation method in which data is aligned with the position of recording mark and the pulse width modulation method in which data is aligned with the length of recording mark. It is known that the pulse width modulation method is superior to the pulse position modulation method in containing a higher recording density.
Now, these optical data recording media are generally formed on a transparent substrate on which tracking grooves have been formed (Erasable Compact Disc Using Phase Change Optical Media; Technical Digest of Topical Meeting on Optical Data Strage TuA3-1-4 (February, 1991). FIG. 1 shows a conventional optical data recording media of this type. The tracking grooves 2 are formed on a transparent substrate 1 and a recording layer 3 is formed on the substrate 1. This recording layer 3 changes its optical characteristics depending on the difference of the heat history of heating by the irradiation of the laser light and cooling. And, the laser light 4 is irradiated to the area of the recording layer 3 between the tracking grooves 2, i.e., the area extruding upwards between the tracking grooves 2. In the conventional optical data recording media, the tracking grooves 2 are generally formed a V-shape, and in most case the width of the region extruded upwards between the tracking grooves 2 is wider than that of the concave region of the tracking groove 2.
However, in a case where the pulse width modulation method is used as the modulation method in the optical data recording media in which the recording layer 3 is formed on the substrate 1 provided with the V-shaped tracking grooves 2 with a wider extruded region between the tracking grooves 2, there is a problem in that the erase ratio largely depends on the frequency of recording laser output. That is, if it is intended to overwrite a shorter recording mark by a high frequency on the area where a longer recording mark has been formed by a low frequency, there is a problem that the erase ratio is lower than that of the opposite case and it is difficult to record signals using the pulse width modulation method. One of the causes creating such problem is that the width of the longer recording mark formed by the low frequency is wider than that of the recording mark formed by the high frequency. That is, in a case where the narrower recording mark by the high frequency is overwritten on the area of recording mark formed by the low frequency, an unerased area remains on the edge of broader recording mark formed by the low frequency, therefore, a high erase ratio can not be obtained.